Both electrical activity and synaptic signals have a pervasive influence on nerve and muscle cells and regulate both the performance of the adult nervous system and its differentiation during development. This project is concerned with the mechanisms underlying innervation-dependent regulation of genes in electrically excitable cells. In skeletal myofibers, innervation regulates two independent transcriptional pathways. One pathway is triggered by signal(s) that are associated with myofiber depolarization, and these signals acts to repress expression of acetylcholine receptor (AChR) genes in nuclei throughout the myofiber. A second pathway is triggered by an unknown signal in the synaptic basal lamina, and this signal acts locally to activate AChR gene expression only in nuclei within the synaptic region. This project is designed to identify the cis-acting regulatory elements in the AChR delta subunit gene that are necessary for each of these innervation-dependent pathways and to characterize the steps involved in coupling innervation to changes in gene expression. These steps include identification of the key cis-acting elements in the AChR delta subunit gene that are necessary for innervation-dependent gene expression, identification of the signal in the synaptic basal lamina that activates synapse-specific transcription, study of the potential role of myogenic bHLH proteins in mediating electrical activity-dependent gene expression, and further characterization of a tyrosine kinase receptor, which is a candidate receptor for the basal lamina signal. Thus, the experiments described in this proposal are likely to provide a basis for a molecular understanding of how genes in electrically excitable cells are regulated by innervation. Moreover, a detailed understanding of how innervation regulates gene expression in the experimentally accessible and relatively simple vertebrate neuromuscular junction is likely to provide important information regarding the steps and mechanisms involved in controlling use-dependent changes in the structure and function of the central and peripheral nervous systems. Further, although the synaptic basal lamina is known to have an important role in directing presynaptic and postsynaptic differentiation, the synaptic basal lamina has not previously been considered to contain molecules that regulate transcription of genes encoding synaptic proteins, and the experiments described in this proposal are likely to provide important information regarding how a signalling molecule(s) in the basal lamina activates expression of genes encoding synaptic proteins.